Plasma etching and particularly photoresist plasma etching (sometimes called photoresist stripping or ashing) in the fabrication of circuits is attractive because it can be chemically selective over different type of layers used in wafer and glass substrate fabrication. Dry (plasma) etch is also more attractive than wet etch, since important environmental issues are associated with the operation of wet chemical processing equipment. The high DI water and wet chemical consumption rate of wet etch approaches represent a large direct chemical and utilities cost. In addition, disposal of resultant large volumes of highly corrosive liquid chemical waste is both costly and hazardous.
A significant opportunity for further expansion in the use of Dry (Plasma) Etch and particularly Photoresist Etching exists in Flat Panel Display (FPD) manufacturing. Etching of the photoresist in FPD manufacturing has historically relied on wet chemical processes. The inherent limitations of wet etch are often exacerbated by unique challenges of wet processing for large-area FPD substrates, which have reached 550.times.650 mm dimensions in the newest manufacturing lines. Since the size of Flat Panel Display substrates is usually significantly larger than that encountered in wafer manufacturing, both the cost and hazard of wet etch in FPD manufacturing are substantially bigger and demand for substitution of wet etch with dry etch techniques is very high. Such substitutions are progressing slowly because of the absence of reliable and inexpensive large area plasma sources.
Plasma Discharge Regimes
We must distinguish two regimes of plasma discharge: low- and high-pressure discharges.
Low-Pressure Discharge
The light and heavy charged particles in low-pressure discharges are almost never in thermal equilibrium, either between themselves or with their surroundings. Because these discharges are electrically driven and are weakly ionized, the applied power preferentially heats the mobile electrons, while the heavy ions efficiently exchange energy by collisions with the background gas. Hence, electron temperature (T.sub.e) is much greater than ion temperature (T.sub.i) for these plasmas. The pressure of these discharges is low and in the range of: p.apprxeq.1 mTorr-10 Torr. Low pressure discharges are characterized by electron temperature T.sub.e.apprxeq.1-10 eV, T.sub.i &lt;&lt;T.sub.e, and ion densities n.sub.i.apprxeq.10.sup.8 -10.sup.13 cm.sup.-3 (see, for example, M. A. Lieberman and A. J. Lichtenberg, "Principles of Plasma Discharges and Materials Processing", John Willey & Sons, 1994). The ion temperature, T.sub.i, usually does not exceed a few times room temperature, e.g. T.sub.i.apprxeq.0.026 eV. In low-pressure discharges feedstock gases are broken into positive ions and chemically reactive etchants, which then flow to and physically and/or chemically react at the substrate surface. While energy is delivered to the substrate also, e.g. in the form of bombarding ions, the energy flux is there to promote the chemistry at the substrate, and not to heat the substrate. Since the heavy particles in a low-pressure plasma are relatively cold, heat flux to the wafer is small and heat management in this case is not an issue (as long as the wafer is remote from the plasma and has negligible voltage near substrate, i.e. bias). Although electron density may be three to five orders of magnitude lower than gas density, the electrons play central roles in sustaining the discharge and in processing at low pressure. Because T.sub.e &gt;&gt;T.sub.i, it is electrons that dissociate the feedstock gases to create the free radicals, etchant atoms, and deposition precursors, required for the chemistry at the substrate. Electrons also ionize the gas to create the positive ions that subsequently bombard the substrate.
A plasma consists of two qualitatively different regions: a quasineutral (n.sub.i.apprxeq.n.sub.e), equipotential conductive plasma body and boundary layer, called the plasma sheath. The plasma body consists of substantially equal densities of negative and positive charged particles as well as radicals and neutral particles. The plasma sheath is an electron deficient, poorly conductive region in which the gradient in the space potential (self-bias) is large. The plasma sheath forms between the plasma and any surrounded surface such as the substrate and walls of plasma chamber. This plasma sheath is useful for anisotropic etch processing. Anisotropic etch enables the production of IC features having sidewalls that are perpendicular to the plane of the photoresist layer. To enhance such anisotropy in modem ULSI (Ultra Large Scale Integration) technology additional power (usually RF) is applied to the substrate (RF bias). The existence of self-bias and the dc component of RF bias leads to substantial kinetic energy in the ions bombarding the substrate. For these ions, bombarding energy can be as high as ten times the electron temperature and a hundred times the T.sub.e for RF bias, even though the ion temperature is two orders of magnitude lower than the electron temperature. Unfortunately, for many IC structures (e.g. thin gate dielectric, etc.) such energy is highly damaging and has become a significant problem in modem ULSI technology. For many isotropic etch processes such high energy bombardment is not required or recommended. The most important such process is photoresist etching. In this process plasma is used for dissociation of oxygen molecules; the products of dissociation, oxygen atoms, are used for ashing organic photoresist. In many other etch processes (so called soft or non-critical etch) neutral free radicals are used for etching and ion bombardment only increase the etch rate, in part by increasing the temperature of the substrate. In such cases high sheath potential and high ion bombardment energy (which result from high electron temperature) are damaging for IC processing. To avoid this damage in modem systems, such as those used for photoresist stripping, specially designed grids are inserted between the plasma source and the wafer. These grids are intended to stop plasma from flowing down to the wafer, but allow free radicals to go through and chemically etch photoresist. Despite these precautions, charge build-up still occurs on the wafer, and the use of a grid significantly reduces ash rate (by about 50%) and contributes to the degradation of system performance over time. The requirement for low damage photoresist stripping shifted oxygen ashers from diode-type to downstream plasma ashers. The technologies currently available include downstream microwave, RF diodes and inductive sources, and even dual mode microwave and RF downstream oxygen plasma ashers.
Problem associated with low pressure plasma photoresist removal include: 1) post-etch "sidewall polymers" that are formed during the reactive plasma etch (RIE); 2)post-etch "via veils"; 3) polysilicon "sidewall stringers"; 4) metal etch residues, and 5) ion implant residues. Without going into the details of formation of these residues, we will note that the usual approach to dealing with the first three residues is to add a short step (about 15s) in the end of the ashing process which involves Oxygen/Fluorine ashing (F&lt;2%). Metal etch residues can be decreased or, in some cases, removed by adding a small amount of halogens.
High dose ion implants create a hardened crust on top of the photoresist mask that is not only difficult to remove in a low pressure plasma discharge, but effective at blocking the escape of vapors from the underlying resist. Using standard low pressure plasma strip techniques to remove the crust will heat up these vapors and cause miniature explosions, often called "poppers". One approach to dealing with this hardened crust is to use a longer, lower temperature ashing process, which removes the crust, followed by a standard resist removal step and final clean-up.
High-Pressure Discharge
High-pressure discharges are also used for processing. These discharges have electron temperature T.sub.e.apprxeq.0.1-1 eV and density n.sub.i.apprxeq.10.sup.14 -10.sup.19 cm.sup.-3, and the light and heavy particles are more nearly in the thermal equilibrium, with T.sub.e.apprxeq.T.sub.i. For those reasons such plasmas are often called thermal plasmas. These discharges are used mainly to heat the substrate, to melt, sinter, or evaporate materials, or to weld or cut refractory materials. Operating pressures are typically near atmospheric pressure (760 Torr). Distinct difference between low- and high-pressure discharges can be seen from the FIG. 1, where electron and gas temperature as function of pressure are shown for an arc discharge.
As we an see from the FIG. 1, an atmospheric pressure plasma has a temperature in the range of 4000.degree. K. to 10000.degree. K., which greatly exceeds any allowed temperature for semiconductor circuits. Careful thermal management is required to employ these plasmas in device processing. This was a reason why atmospheric pressure plasma has not heretofore found a role in ULSI processing.
Atmospheric (thermal) plasma has been mainly produced by DC (sometimes RF) electrical arc discharge or by an Inductively Coupled Plasma Torch. A problem of using an arc discharge in device manufacturing, in addition to the thermal management issues noted above, is erosion of the arc's electrodes which leads to contamination of devices. High pressure arcs cannot be sustained without copious emission of electrons from the cathodes, and this is accomplished by two mechanisms: thermionic emission and field emission. The cathode spots are one or more points of plasma attachment of high current density (on the order of 500-1000 A/cm.sup.2) where the cathode material is very hot (typically in the range from 2000 to 3000.degree. C.). The cathode spots usually move over the cathode surface with a velocity of the order of a meter per second. The anode spot is a single "hot spot", and the anode gets higher heat loads than the cathode. These higher total heat loads can lead to anode burnout. To prevent erosion of electrodes a high heat transfer rate (on the order of 5 kW/cm.sup.2) must be established. Since the requirements on metal contamination in IC processing are very severe, high pressure arcs have not been used, up to now, in front-side IC processing.
Inductive Plasma Torches have been extensively used in spectrochemical analysis as an emission source and in manufacturing of optical fiber and fused silica.
A commercial system with atmospheric pressure arc plasma generation has been developed at the Moscow Institute of Electronic Engineering for backside etching and surface cleaning (see, for example, G. Pavlov et al, Appl. Physics A, v.63, p.9, 1996). A similar system was later commercialized by IPEC Precision (see, for example, O. Siniaguine, Proc. of 1st Plasma Process-Induced Damage Symposium, p. 151, 1996). A plasma arc funnel is produced in this system by applying DC power to two water cooled metal electrodes. The plasma-forming gas is argon and it is injected through both electrodes. The reactants (fluorine and chlorine containing gasses, oxygen, etc.) are injected between electrodes. The argon and reactants create a thermal plasma jet which may be used for backside etching. Since the plasma jet is much smaller than the wafer size, backside etching is produced by multiple consecutive wafer passes over the jet. To control wafer temperature and etch uniformity, the wafer is precisely moved by a computer-controlled drive. The problems associated with such systems are: 1) electrode erosion and wafer contamination (as we discussed above), 2) the small local area of plasma etching requires precise overlap of consecutive passes to have a reasonable etch uniformity, limiting extendibility of this approach to new processes or different substrate sizes. Cylindrical inductive torches are well known in the art, but have not been applied to semiconductor processing heretofore. For small substrates a conventional cylindrical torch may be employed by appropriate motion of the substrate relative to the torch. For larger substrates, the time of processing will become excessively long using g a small diameter cylindrical torch. Simple scaling of cylindrical torches to large areas (for example, 200 mm diameter) would require excessive RF power for excitation, since the power scales as roughly the square of the diameter.
Accordingly, it is desirable to provide an improved plasma generation and processing system.